The present invention relates generally to cordless telephone systems, and more specifically to a cordless key telephone system covering a plurality of service zones which may partially overlap, forming interference-affected zones due to the assignment of a single control channel to adjacent service zones.
Conventional cordless key telephone systems comprise a main unit having first terminals connected to a public or local switched network and second terminals connected to access units located at strategic points of service zones. Each access unit exchanges call-processing signals over control channels with cordless stations and the main unit during call setup times and establishing a speech channel to a cordless station when entering a talking mode. The main unit includes a switch matrix for establishing a switched connection between the network and an appropriate access unit to link the cordless station to the network. During an idle state, each cordless station monitors the control channels assigned to the system. On receiving an incoming call from the network, the main unit selects an access unit for each service zone and causes an alert signal to be broadcast on a control channel assigned to the zone of the selected access unit to elicit an acknowledgment response from cordless stations located in that zone. On receiving that response, the access unit proceeds to select a speech channel and causes the responding stations to switch to the speech channel, whereupon the access units, to which the acknowledgment signals have been returned, broadcast ringing signals on respective speech channels. Since the number of control channels allotted to the system is limited, the channel switching prior to the transmission of a ringing signal is to prevent the control channel from being tied up for a long time before the incoming call is answered. On the other hand, access units are usually located so that the marginal portions of adjacent service zones overlap each other to cover the entire area of a floor space. Care is usually taken to avoid usage of the same control channel by adjacent zones. However, due to the limited number of control channels, some of the overlapping zones may be free from interference, but in other areas interference cannot be avoided. If a cordless station happens to be located in the interference-affected zone, interference occurs between alert signals sent from the adjacent service zones and that cordless station fails to respond to it and receives no ringing signal.